Motors are commonly used to power the operation of a wide range of devices from very small scale machines to much larger assemblies such as elevators (also known as “lifts”). One of the most common forms of motor used, particularly in industrial applications requiring a constant motor speed, are synchronous motors. Synchronous motors synchronise the rotation of a shaft of the motor with a frequency of an AC electrical supply used to power the motor.
In general terms, a synchronous motor comprises a stator and a rotor. The stator includes a number of coils or windings through which electric currents can be fed. The rotor comprises at least one pair of permanent magnets. When an AC current is fed through a winding of the stator, the winding generates a changing magnetic field. Therefore, in a three-phase motor, when the three-phase components of a three-phase AC current are fed through three respective windings, a rotating magnetic field is created in the stator. The rotating magnetic field created in the stator causes rotation of the rotor, and the speed of rotation of the rotor is synchronous with the frequency of the three-phase AC current. The angle between the rotor and the stator produces a resultant net torque, which dictates the net rotational movement of the rotor.
In order for the net rotational movement of the rotor to be in a desired direction and at a desired speed at any given time, the net torque of the rotor must be controlled. The position and phase at which current is injected through the windings in the stator relative to the permanent magnets in the rotor will determine the configuration of the magnetic flux produced by the stator. This will affect the rotational movement imparted by the winding on the rotor, which in turn determines the net torque on the rotor, and therefore the efficiency of the motor.
It is common practice to control the motor by having various speed and position sensors to monitor the rotation of the rotor and adjust characteristics of the voltage applied to the stator in order to control the rotors rotation. However, such methods are extremely computationally expensive and require additional electronic components, which thereby increases the system manufacturing cost.
One means for efficiently controlling a motor, without using speed and position sensors, is by means of a closed current loop (CL) vector control method. Such methods employ techniques, such as signal injection, to determine the rotor shaft position. However, even though these techniques do not use speed and position sensors, they still require considerable computation and rely on high bandwidth current measurement systems.
One alternative means for controlling a motor, particularly for low dynamic motor applications (e.g. pumps and fans), is an open current loop (OL) method. In OL methods, a machine terminal voltage demand is produced based on a profile related to the required mechanical speed. While these OL methods can be implemented on lower cost hardware compared to CL methods and offer the advantage of being more computationally efficient, these methods are extremely inefficient.